Plant hormones

• Plants use hormones to respond to light and gravity
• Phototropism: plant shoots grow towards the light
• Experiments on phototropism:

  • Scientists removed the very tips of shoots, shoots did not grow towards the light
  • Shoot tips produce a plant hormone called auxin
  • Tips of shoots are sensitive to light
  • Lower parts of shoots are not sensitive to light
  • Auxin triggers cell growth at the tip of the shoot
  • Light causes auxin to concentrate on the darker side of the shoot tip
  • Auxin spreads down the shoot, causing cells on the darker side to grow faster than cells on the lighter side, leading the shoot to grow towards the light
• Gravitropism or geotropism: plant roots grow towards the force of gravity

  • Auxin is produced in the root
  • Gravity causes auxin to concentrate on the lower side of the root
  • Auxin inhibits cell growth on the lower side, causing the upper side to grow more quickly and the roots to grow towards gravity
• Other plant hormones:

  • Gibberellins: important in starting the germination of seeds
  • Ethylene: controls cell division and the ripening of fruits
• Plants respond to stimuli in their environment by using hormones, such as auxins, to control growth at the ends of shoots and roots
• Plants can sense light and grow their shoots towards it to maximize photosynthesis
• Plants can sense gravity and grow their roots downwards to fix themselves into the soil and find more water and minerals
• Some plants can sense touch and climb around other objects
• Auxins are plant hormones that control growth at the ends of shoots and roots
• Auxins stimulate growth in shoots but inhibit growth in roots
• Auxin promotes cell division at the root tip.
• Auxins accumulate on the shaded side and lower side of the plant
• Phototropism is a response to light where shoots grow towards the light (positively phototropic)
• Geotropism (or gravitropism) is a response to gravity where shoots grow away from the ground (negatively geotropic) and roots grow downwards towards the ground (positively gravitropic)
• In shoots, auxins cause cells on the shaded side to grow faster than on the sunny side, leading to the shoot curving towards the light
• In roots, auxins cause cells on the lower shaded side to grow faster than on the upper side, leading to the root curving downwards
• The main function of auxin is to promote cell elongation.